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An ambulance rushes to the scene of a bus explosion in the center of the capital, Tunis, Tunisia, Tuesday, Nov. 24, 2015.

An explosion struck a bus carrying members of Tunisia's presidential guard in the country's capital Tuesday, killing at least 12 people and wounding more than a dozen others in what the government described as a terrorist attack.

The blast on a tree-lined avenue in the heart of Tunis is a new blow to a country that is seen as a model for the region but has struggled against Islamic extremist violence. Radical gunmen staged two attacks earlier this year that killed 60 people, devastated the tourism industry and rattled this young democracy.

President Beji Caid Essebsi, who wasn't in the bus at the time of the attack, declared a 30-day state of emergency across the country and imposed an overnight curfew on the Tunis region. He convened an emergency meeting of his security council Wednesday morning.

Speaking on national television, he said Tunisia is at "war against terrorism" and urged international cooperation against extremists who have staged several deadly attacks in recent weeks.

"I want to reassure the Tunisian people that we will vanquish terrorism," he said.

No one immediately claimed responsibility for the attack against the presidential guard, an elite security force that protects only the president.

Police fanned out throughout central Tunis after Tuesday's explosion, and ambulances rushed to the scene, evacuating wounded and dead. Prime Minister Habib Essid and Interior Minister Najem Gharsalli visited the scene of the attack after it was cordoned off by police.

The attack came days after authorities visibly but inexplicably increased the security level in the capital and deployed security forces in unusually high numbers. The French Embassy is this former French territory was also put under high protection in recent days, after extremist attacks in Paris killed 130.

Earlier this month, Tunisian authorities announced the dismantling of a cell that it said had planned attacks at police stations and hotels in the seaside city of Sousse, about 150 kilometers (95 miles) southeast of Tunis. Sousse was one of the targets of attacks earlier this year.

Tunisia's tourism industry has been hit hard this year following extremist attacks. Shootings at a luxury beach hotel in Sousse last June killed 38 people, mostly tourists, while in March, an attack by Islamist extremists at Tunisia's famed Bardo museum near the capital killed 22 people.

U.S. Secretary of State John Kerry, visiting Tunis earlier this month, pledged expanded economic and security support for Tunisia, whose popular uprising unleashed democracy movements across the region in 2011 known as the Arab Spring.

Kerry said the U.S. and Tunisia would soon begin negotiations on a major loan guarantee and were discussing expanded military cooperation, including intelligence sharing and the possible use of drones to collect information about potential threats. A U.S. military team was expected in Tunisia around late November to begin those talks.

Tunisia is the only Arab Spring country to have solidified a new democracy, but it is facing serious economic and security challenges.